Resonant power transfer systems having efficiency optimization based on receiver impedance

ABSTRACT

The present disclosure provides systems and methods for controlling wireless power transfer systems. A wireless power transfer system includes a transmitter driven by a power source and a transmit controller, wherein the transmitter is configured to control delivery of wireless power, and a receiver inductively coupled to the transmitter, the receiver configured to receive the wireless power from the transmitter and deliver the received wireless power to a load. The receiver includes receiver electronics configured to determine a Thevenin equivalent impedance of the wireless power transfer system, determine a Thevenin equivalent source voltage of the wireless power transfer system, and control, based on the determined Thevenin equivalent impedance and the determined Thevenin equivalent source voltage, an ideal source voltage of the receiver to vary the amount of the wireless power transferred from the transmitter to the receiver.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/286,930, filed on Oct. 6, 2016, which claims priority to U.S.Provisional Patent Application No. 62/238,586, filed Oct. 7, 2015, bothof which are hereby incorporated by reference in their entirety.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

FIELD

The field relates generally to resonant wireless power transfer systems,and more specifically to communication systems and methods forimplantable resonant wireless power transfer systems.

BACKGROUND

Many types of devices require transmitting energy between locations.Recent advances have accelerated the pace of innovation for wirelessenergy transmission (WET) without the use of cords. An example of asystem using wireless energy technology is a powered, implantablemedical device.

Many implantable medical devices require electrical systems to power theimplant. Typically, this is achieved using percutaneous wiring toconnect a power source to the implant. More recently, there has beeninterest in development of Transcutaneous Energy Transfer (TET) systems,e.g., through an oscillating magnetic field, for powering implantablemedical devices.

A TET system usually includes a number of components or systems. Aconventional TET system is implemented with a transmitting coil and areceiving coil for transmitting energy across the skin layer. The systemtypically includes a controller for driving the transmitting coil and/orcontrolling the implanted electronics.

Typically, implantable medical devices, such as implanted sensors,require very little power to operate. With such low power levels (on theorder of milliwatts), power transfer levels and efficiency can be lower.With higher power devices (e.g., on the order of watts and up to 15 W ormore), efficient transfer of wireless power is extremely important.Additionally, positions within the body are limited that can accommodatelarger implanted devices, some of which are deep below the skin surface.These implant locations require additional attention to position andorientation of both the transmit and receive coils, as well astechniques to improve and maximize transfer efficiency.

Previous TET systems for implantable medical devices required theimplanted receiver coil to be positioned just under the skin, andtypically include a mechanical feature to align the receive and transmitcoils and keep them together. By implanting these devices directly underthe skin, the size and power requirements of these implanted devices islimited if they are to be powered by a TET system. TET systems can bedesigned for operation even while power is not being received by thereceiver coil. In a typical configuration, solid-state electronics and abattery can power the implanted medical device when external power isinterrupted or not available.

For higher power devices, efficiency and heat losses become veryimportant. Heat loss in an implanted device is limited by the acceptabletemperature rise of the surrounding tissue. Typically a 2° C.temperature rise in tissue is the acceptable limit before riskingthermal damage to the surrounding tissue. This is a relatively smalltemperature rise, meaning the acceptable heat loss in an implant is alsosmall. As a result, the power efficiency of a TET system is veryimportant.

BRIEF DESCRIPTION

In one embodiment, the present disclosure is directed to a wirelesspower transfer system. The wireless power transfer system includes atransmitter driven by a power source and a transmit controller, whereinthe transmitter is configured to control delivery of wireless power, anda receiver inductively coupled to the transmitter, the receiverconfigured to receive the wireless power from the transmitter anddeliver the received wireless power to a load. The receiver includesreceiver electronics configured to determine a Thevenin equivalentimpedance of the wireless power transfer system, determine a Theveninequivalent source voltage of the wireless power transfer system, andcontrol, based on the determined Thevenin equivalent impedance and thedetermined Thevenin equivalent source voltage, an ideal source voltageof the receiver to vary the amount of the wireless power transferredfrom the transmitter to the receiver.

In another embodiment, the present disclosure is directed to a receiverfor use in a wireless power transfer system. The receiver is configuredto receive wireless power from a transmitter driven by a power sourceand a transmit controller, and configured to deliver the receivedwireless power to a load. The receiver includes a receiving coil, andreceiver electronics coupled to the receiving coil. The receiverelectronics are configured to determine a Thevenin equivalent impedanceof the wireless power transfer system, determine a Thevenin equivalentsource voltage of the wireless power transfer system, and control, basedon the determined Thevenin equivalent impedance and the determinedThevenin equivalent source voltage, an ideal source voltage of thereceiver to vary the amount of the wireless power transferred from thetransmitter to the receiver.

In yet another embodiment, the present disclosure is directed to amethod for controlling a wireless power transfer system. The wirelesspower transfer system includes a receiver inductively coupled to atransmitter, the receiver configured to receive wireless power from thetransmitter and deliver the received wireless power to a load. Themethod includes determining, using receiver electronics at the receiver,a Thevenin equivalent impedance of the wireless power transfer system,determining, using the receiver electronics, a Thevenin equivalentsource voltage of the wireless power transfer system, and controlling,using the receiver electronics, based on the determined Theveninequivalent impedance and the determined Thevenin equivalent sourcevoltage, an ideal source voltage of the receiver to vary the amount ofthe wireless power transferred from the transmitter to the receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the disclosure are set forth with particularity inthe claims that follow. A better understanding of the features andadvantages of the present disclosure will be obtained by reference tothe following detailed description that sets forth illustrativeembodiments, in which the principles of the disclosure are utilized, andthe accompanying drawings of which:

FIG. 1 illustrates a basic wireless power transfer system.

FIG. 2 illustrates the flux generated by a pair of coils.

FIGS. 3A-3B illustrate the effect of coil alignment on the couplingcoefficient.

FIG. 4 illustrates a wireless power transfer system comprising animplantable TETS receiver unit implanted in an abdomen of a humanpatient.

FIG. 5 illustrates one embodiment of a TET system 5, including areceiver and a transmitter.

FIG. 6 illustrates the circuit of FIG. 5 modeled as a two port network.

FIG. 7 shows the two port network of FIG. 6 modeled as a Theveninequivalent circuit.

FIG. 8 shows operation of the circuit of FIG. 5 as illustrated by aphasor diagram.

FIG. 9 shows a diagram of a control system for a receiver according toone embodiment of the present disclosure.

FIG. 10 is a block diagram of one embodiment of a computing device thatmay be used to implement receiver electronics.

FIG. 11 is a flow chart of one embodiment of a method for controllingoperation of a wireless power transfer system.

DETAILED DESCRIPTION

In the description that follows, like components have been given thesame reference numerals, regardless of whether they are shown indifferent embodiments. To illustrate an embodiment(s) of the presentdisclosure in a clear and concise manner, the drawings may notnecessarily be to scale and certain features may be shown in somewhatschematic form. Features that are described and/or illustrated withrespect to one embodiment may be used in the same way or in a similarway in one or more other embodiments and/or in combination with orinstead of the features of the other embodiments.

Various aspects of the disclosure are similar to those described inInternational Patent Pub. No. WO2012045050; U.S. Pat. Nos. 8,140,168;7,865,245; 7,774,069; 7,711,433; 7,650,187; 7,571,007; 7,741,734;7,825,543; 6,591,139; 6,553,263; and 5,350,413; and U.S. Pub. Nos.2010/0308939; 2008/0027293; and 2010/0102639, the entire contents ofwhich patents and applications are incorporated herein for all purposes.

Wireless Power Transmission System

Power may be transmitted wirelessly by magnetic induction. In variousembodiments, the transmitter and receiver are closely coupled.

In some cases “closely coupled” or “close coupling” refers to a systemthat requires the coils to be very near each other in order to operate.In some cases “loosely coupled” or “loose coupling” refers to a systemconfigured to operate when the coils have a significant spatial and/oraxial separation, and in some cases up to distance equal to or less thanthe diameter of the larger of the coils. In some cases, “looselycoupled” or “loose coupling” refers a system that is relativelyinsensitive to changes in physical separation and/or orientation of thereceiver and transmitter.

In some cases “loosely coupled” or “loose coupling” refers to a systemconfigured to operate when the coils have a significant spatial and/oraxial separation, and in some cases up to distance equal to or less thanthe diameter of the larger of the coils. In some cases, “looselycoupled” or “loose coupling” refers a system that is relativelyinsensitive to changes in physical separation and/or orientation of thereceiver and transmitter. In some cases, “loosely coupled” or “loosecoupling” refers a highly resonant system and/or a system usingstrongly-coupled magnetic resonators.

In various embodiments, the transmitter and receiver are non-resonantcoils. For example, a change in current in one coil induces a changingmagnetic field. The second coil within the magnetic field picks up themagnetic flux, which in turn induces a current in the second coil. Anexample of a closely coupled system with non-resonant coils is describedin International Pub. No. WO2000/074747, incorporated herein for allpurposes by reference. A conventional transformer is another example ofa closely coupled, non-resonant system. In various embodiments, thetransmitter and receiver are resonant coils. For example, one or both ofthe coils is connected to a tuning capacitor or other means forcontrolling the frequency in the respective coil. Examples of closelycoupled system with resonant coils are described in International Pub.Nos. WO2001/037926; WO2012/087807; WO2012/087811; WO2012/087816;WO2012/087819; WO2010/030378; and WO2012/056365, U.S. Pub. No.2003/0171792, and U.S. Pat. No. 5,350,413 (now abandoned), incorporatedherein for all purposes by reference.

For given coil sizes and separations, coupling a given amount of powerrequires generating the same magnetic field strength for eitherinductive or resonant systems. This requires the same number ofampere-turns in the coils. In inductive systems, all the ampere-turnspass through the MOSFETs and generate power losses in their on-stateresistance. In resonant systems, only the exciter ampere-turns passthrough the MOSFETs, while the resonator ampere-turns do not. As aconsequence, resonant systems will always have lower losses and higherefficiencies than inductive systems of the same dimensions and powerthrough-put.

In various embodiments, the transmitter and receiver are non-resonantcoils. For example, a change in current in one coil induces a changingmagnetic field. The second coil within the magnetic field picks up themagnetic flux, which in turn induces a current in the second coil. Anexample of a closely coupled system with non-resonant coils is describedin International Pub. No. WO2000/074747, incorporated herein for allpurposes by reference. A conventional transformer is another example ofa closely coupled, non-resonant system. In various embodiments, thetransmitter and receiver are resonant coils. For example, one or both ofthe coils is connected to a tuning capacitor or other means forcontrolling the frequency in the respective coil. An example of closelycoupled system with resonant coils is described in International Pub.Nos. WO2001/037926; WO2012/087807; WO2012/087811; WO2012/087816;WO2012/087819; WO2010/030378; and WO2012/056365, and U.S. Pub. No.2003/0171792, incorporated herein for all purposes by reference.

In various embodiments, the transmitter and receiver are looselycoupled. For example, the transmitter can resonate to propagate magneticflux that is picked up by the receiver at relatively great distances. Insome cases energy can be transmitted over several meters. In a looselycoupled system power transfer may not necessarily depend on a criticaldistance. Rather, the system may be able to accommodate changes to thecoupling coefficient between the transmitter and receiver. An example ofa loosely coupled system is described in International Pub. No.WO2012/045050, incorporated herein for all purposes by reference.

Power may be transmitted wirelessly by radiating energy. In variousembodiments, the system comprises antennas. The antennas may be resonantor non-resonant. For example, non-resonant antennas may radiateelectromagnetic waves to create a field. The field can be near field orfar field. The field can be directional. Generally far field has greaterrange but a lower power transfer rate. An example of such a system forradiating energy with resonators is described in International Pub. No.WO2010/089354, incorporated herein for all purposes by reference. Anexample of such a non-resonant system is described in International Pub.No. WO2009/018271, incorporated herein for all purposes by reference.Instead of antenna, the system may comprise a high energy light sourcesuch as a laser. The system can be configured so photons carryelectromagnetic energy in a spatially restricted, direct, coherent pathfrom a transmission point to a receiving point. An example of such asystem is described in International Pub. No. WO2010/089354,incorporated herein for all purposes by reference.

Power may also be transmitted by taking advantage of the material ormedium through which the energy passes. For example, volume conductioninvolves transmitting electrical energy through tissue between atransmitting point and a receiving point. An example of such a system isdescribed in International Pub. No. WO2008/066941, incorporated hereinfor all purposes by reference.

Power may also be transferred using a capacitor charging technique. Thesystem can be resonant or non-resonant. Exemplars of capacitor chargingfor wireless energy transfer are described in International Pub. No.WO2012/056365, incorporated herein for all purposes by reference.

The system in accordance with various aspects of the invention will nowbe described in connection with a system for wireless energy transfer bymagnetic induction. The exemplary system utilizes resonant powertransfer. The system works by transmitting power between the twoinductively coupled coils. In contrast to a transformer, however, theexemplary coils are not coupled together closely. A transformergenerally requires the coils to be aligned and positioned directlyadjacent each other. The exemplary system accommodates looser couplingof the coils.

While described in terms of one receiver coil and one transmitter coil,one will appreciate from the description herein that the system may usetwo or more receiver coils and two or more transmitter coils. Forexample, the transmitter may be configured with two coils—a first coilto resonate flux and a second coil to excite the first coil. One willfurther appreciate from the description herein that usage of “resonator”and “coil” may be used somewhat interchangeably. In various respects,“resonator” refers to a coil and a capacitor connected together.

In accordance with various embodiments of this disclosure, the systemcomprises one or more transmitters configured to transmit powerwirelessly to one or more receivers. In various embodiments, the systemincludes a transmitter and more than one receiver in a multiplexedarrangement. A frequency generator may be electrically coupled to thetransmitter to drive the transmitter to transmit power at a particularfrequency or range of frequencies. The frequency generator can include avoltage controlled oscillator and one or more switchable arrays ofcapacitors, a voltage controlled oscillator and one or more varactors, aphase-locked-loop, a direct digital synthesizer, or combinationsthereof. The transmitter can be configured to transmit power at multiplefrequencies simultaneously. The frequency generator can include two ormore phase-locked-loops electrically coupled to a common referenceoscillator, two or more independent voltage controlled oscillators, orcombinations thereof. The transmitter can be arranged to simultaneouslydelivery power to multiple receivers at a common frequency.

In various embodiments, the transmitter is configured to transmit a lowpower signal at a particular frequency. The transmitter may transmit thelow power signal for a particular time and/or interval. In variousembodiments, the transmitter is configured to transmit a high powersignal wirelessly at a particular frequency. The transmitter maytransmit the high power signal for a particular time and/or interval.

In various embodiments, the receiver includes a frequency selectionmechanism electrically coupled to the receiver coil and arranged toallow the resonator to change a frequency or a range of frequencies thatthe receiver can receive. The frequency selection mechanism can includea switchable array of discrete capacitors, a variable capacitance, oneor more inductors electrically coupled to the receiving antenna,additional turns of a coil of the receiving antenna, or combinationsthereof.

In general, most of the flux from the transmitter coil does not reachthe receiver coil. The amount of flux generated by the transmitter coilthat reaches the receiver coil is described by “k” and referred to asthe “coupling coefficient.”

In various embodiments, the system is configured to maintain a value ofk in the range of between about 0.2 to about 0.01. In variousembodiments, the system is configured to maintain a value of k of atleast 0.01, at least 0.02, at least 0.03, at least 0.04, or at least0.05.

In various embodiments, the coils are physically separated. In variousembodiments, the separation is greater than a thickness of the receivercoil. In various embodiments, the separation distance is equal to orless than the diameter of the larger of the receiver and transmittercoil.

Because most of the flux does not reach the receiver, the transmittercoil must generate a much larger field than what is coupled to thereceiver. In various embodiments, this is accomplished by configuringthe transmitter with a large number of amp-turns in the coil.

Since only the flux coupled to the receiver gets coupled to a real load,most of the energy in the field is reactive. The current in the coil canbe sustained with a capacitor connected to the coil to create aresonator. The power source thus only needs to supply the energyabsorbed by the receiver. The resonant capacitor maintains the excessflux that is not coupled to the receiver.

In various embodiments, the impedance of the receiver is matched to thetransmitter. This allows efficient transfer of energy out of thereceiver. In this case the receiver coil may not need to have a resonantcapacitor.

Turning now to FIG. 1, a simplified circuit for wireless energytransmission is shown. The exemplary system shows a series connection,but the system can be connected as either series or parallel on eitherthe transmitter or receiver side.

The exemplary transmitter includes a coil Lx connected to a power sourceVs by a capacitor Cx. The exemplary receiver includes a coil Lyconnected to a load by a capacitor Cy. Capacitor Cx may be configured tomake Lx resonate at a desired frequency. Capacitance Cx of thetransmitter coil may be defined by its geometry. Inductors Lx and Ly areconnected by coupling coefficient k. Mxy is the mutual inductancebetween the two coils. The mutual inductance, Mxy, is related tocoupling coefficient, k.Mxy=k√{square root over (Lx·Ly)}

In the exemplary system the power source Vs is in series with thetransmitter coil Lx so it may have to carry all the reactive current.This puts a larger burden on the current rating of the power source andany resistance in the source will add to losses.

The exemplary system includes a receiver configured to receive energywirelessly transmitted by the transmitter. The exemplary receiver isconnected to a load. The receiver and load may be connected electricallywith a controllable switch.

In various embodiments, the receiver includes a circuit elementconfigured to be connected or disconnected from the receiver coil by anelectronically controllable switch. The electrical coupling can includeboth a serial and parallel arrangement. The circuit element can includea resistor, capacitor, inductor, lengths of an antenna structure, orcombinations thereof. The system can be configured such that power istransmitted by the transmitter and can be received by the receiver inpredetermined time increments.

In various embodiments, the transmitter coil and/or the receiver coil isa substantially two-dimensional structure. In various embodiments, thetransmitter coil may be coupled to a transmitter impedance-matchingstructure. Similarly, the receiver coil may be coupled to a receiverimpedance-matching structure. Examples of suitable impedance-matchingstructures include, but are not limited to, a coil, a loop, atransformer, and/or any impedance-matching network. Animpedance-matching network may include inductors or capacitorsconfigured to connect a signal source to the resonator structure.

In various embodiments, the transmitter is controlled by a controller(not shown) and driving circuit. The controller and/or driving circuitmay include a directional coupler, a signal generator, and/or anamplifier. The controller may be configured to adjust the transmitterfrequency or amplifier gain to compensate for changes to the couplingbetween the receiver and transmitter.

In various embodiments, the transmitter coil is connected to animpedance-matched coil loop. The loop is connected to a power source andis configured to excite the transmitter coil. The first coil loop mayhave finite output impedance. A signal generator output may be amplifiedand fed to the transmitter coil. In use power is transferredmagnetically between the first coil loop and the main transmitter coil,which in turns transmits flux to the receiver. Energy received by thereceiver coil is delivered by Ohmic connection to the load.

One of the challenges to a practical circuit is how to get energy in andout of the resonators. Simply putting the power source and load inseries or parallel with the resonators is difficult because of thevoltage and current required. In various embodiments, the system isconfigured to achieve an approximate energy balance by analyzing thesystem characteristics, estimating voltages and currents involved, andcontrolling circuit elements to deliver the power needed by thereceiver.

In an exemplary embodiment, the system load power, P_(L), is assumed tobe 15 Watts and the operating frequency of the system, f, is 250 kHz.Then, for each cycle the load removes a certain amount of energy fromthe resonance:

$e_{L} = {\frac{P_{L}}{f} = {60\mspace{14mu}\mu\; J\mspace{14mu}{Energy}\mspace{14mu}{the}\mspace{14mu}{load}\mspace{14mu}{removes}\mspace{14mu}{in}\mspace{14mu}{one}\mspace{14mu}{cycle}}}$

It has been found that the energy in the receiver resonance is typicallyseveral times larger than the energy removed by the load for operative,implantable medical devices. In various embodiments, the system assumesa ratio 7:1 for energy at the receiver versus the load removed. Underthis assumption, the instantaneous energy in the exemplary receiverresonance is 420 μJ.

The exemplary circuit was analyzed and the self inductance of thereceiver coil was found to be 60 uH. From the energy and the inductance,the voltage and current in the resonator could be calculated.

$e_{y} = {\frac{1}{2}{Li}^{2}}$$i_{y} = {\sqrt{\frac{2e_{y}}{L}} = {3.74\mspace{14mu} A\mspace{14mu}{peak}}}$v_(y) = ω L_(y)i_(y) = 352  V  peak

The voltage and current can be traded off against each other. Theinductor may couple the same amount of flux regardless of the number ofturns. The Amp-turns of the coil needs to stay the same in this example,so more turns means the current is reduced. The coil voltage, however,will need to increase. Likewise, the voltage can be reduced at theexpense of a higher current. The transmitter coil needs to have muchmore flux. The transmitter flux is related to the receiver flux by thecoupling coefficient. Accordingly, the energy in the field from thetransmitter coil is scaled by k.

$e_{x} = \frac{e_{y}}{k}$

Given that k is 0.05:

$e_{x} = {\frac{420\mspace{14mu}\mu\; J}{0.05} = {8.4\mspace{14mu}{mJ}}}$

For the same circuit the self inductance of the transmitter coil was 146uH as mentioned above. This results in:

$i_{x} = {\sqrt{\frac{2e_{x}}{L}} = {10.7\mspace{14mu} A\mspace{14mu}{peak}}}$v_(x) = ω L_(x)i_(x) = 2460  V  peak

One can appreciate from this example, the competing factors and how tobalance voltage, current, and inductance to suit the circumstance andachieve the desired outcome. Like the receiver, the voltage and currentcan be traded off against each other. In this example, the voltages andcurrents in the system are relatively high. One can adjust the tuning tolower the voltage and/or current at the receiver if the load is lower.

Estimation of Coupling Coefficient and Mutual Inductance

As explained above, the coupling coefficient, k, may be useful for anumber of reasons. In one example, the coupling coefficient can be usedto understand the arrangement of the coils relative to each other sotuning adjustments can be made to ensure adequate performance. If thereceiver coil moves away from the transmitter coil, the mutualinductance will decrease, and ceteris paribus, less power will betransferred. In various embodiments, the system is configured to maketuning adjustments to compensate for the drop in coupling efficiency.

The exemplary system described above often has imperfect information.For various reasons as would be understood by one of skill in the art,the system does not collect data for all parameters. Moreover, becauseof the physical gap between coils and without an external means ofcommunications between the two resonators, the transmitter may haveinformation that the receiver does not have and vice versa. Theselimitations make it difficult to directly measure and derive thecoupling coefficient, k, in real time.

Described below are several principles for estimating the couplingcoefficient, k, for two coils of a given geometry. The approaches maymake use of techniques such as Biot-Savart calculations or finiteelement methods. Certain assumptions and generalizations, based on howthe coils interact in specific orientations, are made for the sake ofsimplicity of understanding. From an electric circuit point of view, allthe physical geometry permutations can generally lead to the couplingcoefficient.

If two coils are arranged so they are in the same plane, with one coilcircumscribing the other, then the coupling coefficient can be estimatedto be roughly proportional to the ratio of the area of the two coils.This assumes the flux generated by coil 1 is roughly uniform over thearea it encloses as shown in FIG. 2.

If the coils are out of alignment such that the coils are at a relativeangle, the coupling coefficient will decrease. The amount of thedecrease is estimated to be about equal to the cosine of the angle asshown in FIG. 3A. If the coils are orthogonal to each other such thattheta (θ) is 90 degrees, the flux will not be received by the receiverand the coupling coefficient will be zero.

If the coils are arraigned such that half the flux from one coil is inone direction and the other half is in the other direction, the fluxcancels out and the coupling coefficient is zero, as shown in FIG. 3B.

A final principle relies on symmetry of the coils. The couplingcoefficient and mutual inductance from one coil to the other is assumedto be the same regardless of which coil is being energized.M _(xy) =M _(yx)

FIG. 4 illustrates a wireless power transfer system comprising animplantable TETS receiver unit 400 implanted in an abdomen of a humanpatient. The receiver unit 400 can be coupled to a device load 402, suchas an implantable medical device, e.g., an implantable LVAD or heartpump. The exemplary receiver unit 400 can include a receiver resonatorcoil and electronics configured to receive wireless energy from anexternal transmitter 401, which can include a power supply such as apulse generator connected to a transmitter resonator coil. In oneembodiment, the electronics and coils are implanted separately andconnected by an implanted cable. In some embodiments, externalcontroller 404 can be configured to communicate with the TETS receiverunit 400 and can be worn by the patient, such as on the patient's wrist.In other embodiments, the external controller can be an electroniccomputing device such as a personal computer, a tablet, smartphone, orlaptop computer.

In some embodiments, the receiver unit 400 further includescommunications electronics configured to send and receive communicationsdata to and from other electronic devices inside and outside of thebody. In one embodiment, the receiver unit further includes an internalrechargeable power source. In various embodiments, the receiver unit 400of the TET system is configured as a single implanted device includingthe receive coil, antenna, power source, and associated circuitry. Thereceiver unit is configured so the implantable medical device can beplugged directly into the unit. The single housing configuration makesimplantation easier and faster. Additionally, since there are lessimplants, and consequently less tunneling in the body and percutaneousdefect sites, adverse event risks like bleeding and infection arereduced. One of skill will appreciate from the description herein thatvarious internal components of the system can be bundled together orimplanted separately. For example, the internal rechargeable powersource can be implanted separately from the receiver unit and connectedby a power cable. The antenna assembly, power source, and receive coilcan all be configured in separate hermetically sealed housings.International Pub. No. WO2007/053881A1, U.S. Pub. No. 2014/0005466, andU.S. Pat. No. 8,562,508, the entire contents of which are incorporatedherein for all purposes by reference, disclose several exemplaryconfigurations.

TET systems of the present disclosure provide solutions for optimizingefficiency to maximize battery life and to minimize heat in both theimplanted receiver and the external transmitter. The most efficientoperating condition is where the current in the resonant system is assmall as possible. However, if the current is too small, the TET systemwill not be able to transfer enough power. If the TET system tries topull more power out of the resonators than can be supported, theresonant system collapses which results in no power transfer.

According to the present disclosure, the receiver unit can be configuredto monitor the impedance of the transmit and receive resonators andcalculate the amount of power than can be transferred through the TETsystem. The receiver unit can use this calculated amount of power tolimit how much power the receiver unit pulls out of the resonant systemand avoid collapse of the system. If the receiver unit requires morepower than it is pulling from the resonant system, the receiver unit cancommunicate the need for more power to the transmitter unit (e.g., bytransmitting a control message from the receiver to the transmitter),which can then drive the resonators with more current to transferadditional power. This allows the TET system to run at the minimumamount of current required to transfer the desired amount of power,which minimizes losses in the system.

The receiver can communicate with the transmitter using any suitablecommunications techniques. For example, TETs power signals between thetransmitter and receiver may be modulated to communicate informationbetween the transmitter and receiver. Alternatively, a separate radiomay be used to send messages from the receiver to the transmitter.Further, optical or acoustic communication methods may be used as well.U.S. Pub. No. 2015/0222139, which is incorporated by reference herein inits entirety, discussion communications between a receiver and atransmitter in a wireless energy transfer system.

FIG. 5 illustrates one embodiment of a TET system 50, including areceiver 500 and a transmitter 501. The circuit shown in FIG. 5 canfurther modeled as a two port network, shown in FIG. 6. The transmitterpower electronics are modeled as an ideal voltage source, Vx. Thereceiver power electronics are also modeled as an ideal voltage source,Vy. The currents and voltages are described using a phasor notation.

From the point of view of the receiver, the two port network of FIG. 6can be represented by the Thevenin equivalent circuit. The resonantsystem appears to be a voltage source and an impedance, as shown in FIG.7. The impedance Zt is a function of the resonant system's inductanceand capacitance, so it changes as the mutual coupling between thetransmitter and the receiver changes. The relative position of thetransmitter and receiver changes the impedance Zt. The voltage source Vtchanges both magnitude and phase as a function of the mutual coupling,and the transmitter voltage Vx.

The receiver electronics are able to control the voltage source, Vy, toget a specific voltage magnitude and phase. The operation of the circuitis illustrated by the phasor diagram in FIG. 8. This diagram shows thevoltage source Vy is controlled to be in phase with the current Iy. TheTET system is configured to transfer the maximum amount of power whenthe magnitude of Vy is equal to the magnitude of IyZt. To control thepower through the resonators, the circuit can be controlled in a coupleways, including:

Method 1: For many systems the magnitude of Vt and Zt are large. Thereis a limit to the magnitude of Vy. In the system shown in FIG. 5, thelimit is the DC bus voltage of the receiver. If Vt is large relative toVy, the power through the system can be controlled by adjusting themagnitude of Vy. Increasing Vy increases the amount of power through thesystem.

Method 2: In another embodiment, the way to control the amount of poweronly works if the magnitude of Vt is smaller than the largest possiblemagnitude of Vy. If Vy is equal to Vt then no power is transferredthrough the system. As Vy is reduced then the amount of power absorbedby the receiver increases.

To avoid having the receiver try to pull more power than the system canhandle, it is possible to limit the magnitude of the voltage Vy, to bealways less than IyZt for control method 1, or always greater than IyZt.This way the resonance never collapses. If Zt and Iy are known it ispossible to know how close to the power limit the system is operating.If the system is operating too close to the limit, the receiver cancommunicate this information to the transmitter, which can turn up itsoutput, Vx, effectively turning up Vt, so the system can transfer morepower. For example, in some embodiments, if a difference between IyZtand Vy is less than a predetermined threshold, the receiver transmits acontrol message to the transmitter to cause the transmitter to increaseVx. In this way, the transmitter output can be optimized to run at theminimum current needed, minimizing losses, achieving the most efficientoperation possible.

To measure Zt it is possible for the receiver to vary the voltage Vy andobserve the change in Iy. Specifically, if Vy is varied by apredetermined amount (referred to as ‘dither’), in a sampled datasystem, we have:Vy[n]=Vy[n−1]±dither

Further, the Thevenin equivalent impedance is then:

${{Zt}\lbrack n\rbrack} = \frac{{{Vy}\lbrack n\rbrack} - {{Vy}\lbrack {n - 1} \rbrack}}{{{Iy}\lbrack n\rbrack} - {{Iy}\lbrack {n - 1} \rbrack}}$

The values in these equations are phasors. Accordingly, the mathematicaloperations are performed using complex numbers. This method ofcalculating Zt works if the voltage Vt is constant and the value of Ztis constant. This will be true if the voltage Vx is constant and thetransmitter and receiver are not moving relative to one another. Inpractice, it can be assumed that any relative movement of thetransmitter and receiver is very slow compared to the voltage signal Vy.Further, in practice, Vx is controlled, so it does change. However, aslong as the control system driving Vx is slow as compared to the voltagesignal Vy, and any relative movement of the transmitter and receiver isrelatively slow, the measured result of Zt is close enough to the actualvalue to facilitate optimizing power transfer.

FIG. 9 shows a diagram of one embodiment of a control system 900 for areceiver according to one embodiment of the present disclosure. In thisembodiment, control system 900 is implemented using the receiverelectronics. Control system 900 adjusts the magnitude and phase of thevoltage signal Vy in order to achieve a desired current Idcy (shown inFIG. 5). Control system 900 is used when IyZt is larger than Vy, so thatincreasing Vy results in an increase in power received by the receiver.

In this embodiment, the voltage Vy is driven by a MOSFET bridge 502(shown in FIG. 5) that connects the Vdcy and Vy circuits. MOSFET bridge502 is controlled by a pulse width modulated signal from amicrocontroller (e.g., as part of the receiver electronics).Specifically, control system 900 controls a duty cycle and phase of thepulse width modulated signal, resulting in Vy. The phase is controlledsuch that Vy is almost always in phase with Iy. The exception occurswhen the desired voltage is very small (at which point the duty cycle isclamped at a minimum value and control system 900 adjusts the phase ofthe output voltage).

In this embodiment, control system 900 adjusts Vy in order to achieve adescribed Idcy value. The amount of desired current is based on thedemands of the receiver. In this embodiment, control system 900 isimplemented using a proportional integrative (PI) controller.Alternatively, control system 900 may be implemented using any suitablecontroller. The output of the PI controller is limited by v_clamp, asshown in FIG. 9. v_clamp is the maximum allowed voltage and iscalculated by a function called IdcCutback. This function observers theresonator impedance (Zt) and calculates IyZt. In this embodiment, themagnitude of IyZt is the maximum voltage possible for Vy, and IyZt isused for v_clamp. This facilitates limiting the power the receiver canpull from the resonant system to prevent the resonance from collapsing.

In some embodiments, the receiver electronics are implemented as acomputing device. For example, FIG. 10 is a block diagram of oneembodiment of a computing device 1000 that may be used to perform thefunctions of the receiver electronics.

In this embodiment, computing device 1000 includes at least one memorydevice 1010 and a processor 1015 that is coupled to memory device 1010for executing instructions. In some embodiments, executable instructionsare stored in memory device 1010. In the illustrated embodiment,computing device 1000 performs one or more operations described hereinby programming processor 1015. For example, processor 1015 may beprogrammed by encoding an operation as one or more executableinstructions and by providing the executable instructions in memorydevice 1010.

Processor 1015 may include one or more processing units (e.g., in amulti-core configuration). Further, processor 1015 may be implementedusing one or more heterogeneous processor systems in which a mainprocessor is present with secondary processors on a single chip. Inanother illustrative example, processor 1015 may be a symmetricmulti-processor system containing multiple processors of the same type.Further, processor 1015 may be implemented using any suitableprogrammable circuit including one or more systems and microcontrollers,microprocessors, reduced instruction set circuits (RISC), applicationspecific integrated circuits (ASIC), programmable logic circuits, fieldprogrammable gate arrays (FPGA), and any other circuit capable ofexecuting the functions described herein.

In the illustrated embodiment, memory device 1010 is one or more devicesthat enable information such as executable instructions and/or otherdata to be stored and retrieved. Memory device 1010 may include one ormore computer readable media, such as, without limitation, dynamicrandom access memory (DRAM), read-only memory (ROM), electricallyerasable programmable read-only memory (EEPROM), static random accessmemory (SRAM), a solid state disk, and/or a hard disk. Memory device1010 may be configured to store, without limitation, application sourcecode, application object code, source code portions of interest, objectcode portions of interest, configuration data, execution events and/orany other type of data.

Computing device 1000, in the illustrated embodiment, includes acommunication interface 1040 coupled to processor 1015. Communicationinterface 240 communicates with one or more remote devices, such as thetransmitter. To communicate with remote devices, communication interface140 may include, for example, a wired network adapter, a wirelessnetwork adapter, a radio-frequency (RF) adapter, and/or a mobiletelecommunications adapter.

FIG. 11 is a flow chart of one embodiment of a method 1100 forcontrolling a wireless power transfer system. Method 1100 may beimplemented, for example, using computing device 1000 (shown in FIG.10). In this embodiment, method 1100 includes determining 1102 aThevenin equivalent impedance of the wireless power transfer system.Method 1100 further includes determining 1104 a Thevenin equivalentsource voltage of the wireless power transfer system. Method 1100further includes controlling 1106, based on the determined Theveninequivalent impedance and the determined Thevenin equivalent sourcevoltage, an ideal source voltage of the receiver to vary the amount ofthe wireless power transferred from the transmitter to the receiver.

Although various illustrative embodiments are described above, any of anumber of changes may be made to various embodiments without departingfrom the scope of the disclosure as described by the claims. Forexample, the order in which various described method steps are performedmay often be changed in alternative embodiments, and in otheralternative embodiments one or more method steps may be skippedaltogether. Optional features of various device and system embodimentsmay be included in some embodiments and not in others. Therefore, theforegoing description is provided primarily for exemplary purposes andshould not be interpreted to limit the scope of the disclosure as it isset forth in the claims. Although described in some respects as amedical system, one will appreciate from the description herein that theprinciples can apply equally to other types of systems including, butnot limited to, consumer electronics, automotive, phones and personalcommunication devices, gaming devices, and computers and peripherals.

The examples and illustrations included herein show, by way ofillustration and not of limitation, specific embodiments in which thesubject matter may be practiced. As mentioned, other embodiments may beutilized and derived there from, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure. Such embodiments of the inventive subject matter maybe referred to herein individually or collectively by the term“invention” merely for convenience and without intending to voluntarilylimit the scope of this application to any single invention or inventiveconcept, if more than one is, in fact, disclosed. Thus, althoughspecific embodiments have been illustrated and described herein, anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations or variations of various embodiments.Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the above description.

What is claimed is:
 1. A wireless power transfer system comprising: atransmitter driven by a power source and a transmit controller, whereinthe transmitter is configured to control delivery of wireless power; anda receiver inductively coupled to the transmitter, the receiverconfigured to receive the wireless power from the transmitter anddeliver the received wireless power to a load as direct current (DC)power via a converter circuit, the receiver comprising a control systemconfigured to: calculate a Thevenin equivalent impedance of the wirelesspower transfer system from the point of view of the receiver based oncurrent measurements at an input of the converter circuit; calculate amaximum allowed voltage using a cutback function that monitors theThevenin equivalent impedance and calculates the maximum allowed voltagefrom the Thevenin equivalent impedance; and control, using aproportional integrative (PI) controller, the converter circuit tocontrol an ideal source voltage at the input of the converter circuitbased on the calculated Thevenin equivalent impedance to vary an amountof the wireless power transferred from the transmitter to the receiver,wherein an output of the PI controller is limited to the maximum allowedvoltage.
 2. The wireless power transfer system of claim 1, wherein tocontrol the converter circuit to control the ideal source voltage, thecontrol system is configured to increase the ideal source voltage toincrease a DC current associated with the DC power.
 3. The wirelesspower transfer system of claim 1, wherein the converter circuit is aMOSFET bridge, and wherein to control the converter circuit to controlthe ideal source voltage, the control system is configured control theMOSFET bridge to drive the ideal source voltage.
 4. The wireless powertransfer system of claim 3, wherein the control system is configured tocontrol the MOSFET bridge by controlling a duty cycle and a phase of apulse width modulated signal that drives the MOSFET bridge.
 5. Thewireless power transfer system of claim 1, wherein the control system isconfigured to control the converter circuit to control the ideal sourcevoltage to be in phase with an ideal source current of the receiver atthe input of the converter circuit.
 6. A receiver for use in a wirelesspower transfer system, the receiver configured to receive wireless powerfrom a transmitter driven by a power source and a transmit controller,and configured to deliver the received wireless power to a load asdirect current (DC) power via a converter circuit, the receivercomprising: a receiving coil; and a control system coupled to thereceiving coil, the control system configured to: calculate a Theveninequivalent impedance of the wireless power transfer system from thepoint of view of the receiver based on current measurements at an inputof the converter circuit; calculate a maximum allowed voltage using acutback function that monitors the Thevenin equivalent impedance andcalculates the maximum allowed voltage from the Thevenin equivalentimpedance; and control, using a proportional integrative (PI)controller, the converter circuit to control an ideal source voltage atthe input of the converter circuit based on the calculated Theveninequivalent impedance to vary an amount of the wireless power transferredfrom the transmitter to the receiver, wherein an output of the PIcontroller is limited to the maximum allowed voltage.
 7. The receiver ofclaim 6, wherein to control the converter circuit to control the idealsource voltage, the control system is configured to increase the idealsource voltage to increase a DC current associated with the DC power. 8.The receiver of claim 6, wherein the converter circuit is a MOSFETbridge, and wherein to control the converter circuit to control theideal source voltage, the control system is configured control theMOSFET bridge to drive the ideal source voltage.
 9. The receiver ofclaim 8, wherein the control system is configured to control the MOSFETbridge by controlling a duty cycle and a phase of a pulse widthmodulated signal that drives the MOSFET bridge.
 10. The receiver ofclaim 6, wherein control system is configured to control the convertercircuit to control the ideal source voltage to be in phase with an idealsource current of the receiver.
 11. A method for controlling a wirelesspower transfer system, the wireless power transfer system including areceiver inductively coupled to a transmitter, the receiver configuredto receive wireless power from the transmitter and deliver the receivedwireless power to a load as direct current (DC) power via a convertercircuit, the method comprising: calculating, using a control system, aThevenin equivalent impedance of the wireless power transfer system fromthe point of view of the receiver based on current measurements at aninput of the converter circuit; calculating a maximum allowed voltageusing a cutback function that monitors the Thevenin equivalent impedanceand calculates the maximum allowed voltage from the Thevenin equivalentimpedance; and controlling, using a proportional integrative (PI)controller of the control system, the converter circuit to control anideal source voltage at the input of the converter circuit based on thecalculated Thevenin equivalent impedance to vary an amount of thewireless power transferred from the transmitter to the receiver, whereinan output of the PI controller is limited to the maximum allowedvoltage.
 12. The method of claim 11, wherein the converter circuit is aMOSFET bridge, and wherein controlling the converter circuit to controlthe ideal source voltage comprises controlling the MOSFET bridge todrive the ideal source voltage.
 13. The method of claim 12, whereincontrolling the MOSFET bridge comprises controlling a duty cycle and aphase of a pulse width modulated signal that drives the MOSFET bridge.